Diverse methods are known in the prior art for scanning an examination object with two or more different x-ray energy spectra.
In a first simple variant a single-energy source x-ray system is used, which carries out two object scans serially one after the other with different energy spectra, wherein each scan comprises at least one half orbit and a different acceleration voltage is set at the x-ray tube in each case. This method proves problematic however, especially for the scanning of patients, since spatial shifts can occur between the scans and thus a strict spatial correlation is no longer to be insured between the individually determined object data.
A dual-energy source x-ray system is also mostly used for dual-energy spectra scanning, which scans the examination object temporally in parallel with the aid of two x-ray tubes operated with different acceleration voltages and thus delivers two projection datasets per x-ray energy spectrum, from which the image datasets can then be reconstructed. In practice, on grounds of cost, detectors with different spatial extent are used in such dual-energy source x-ray systems, so that one of the datasets is based on a smaller measurement field and corresponding restrictions in the datasets obtained therefrom must be accepted.
As an alternative, scanning methods are also known in which, with the aid of a fast switching of the acceleration voltage using a single mobile emitter-detector arrangement (fast-kV-switching) between the individual readings, temporally parallel projection datasets are determined with different x-ray energies and image datasets are reconstructed therefrom in each case. The problem in such cases is that, while retaining an unchanged tube current independent of the acceleration voltage, drastically different photon flows arise as a function of the acceleration voltage, so that correspondingly drastically different signal-to-noise ratios are also produced in the image data calculated therefrom. In order to solve this problem it has also been proposed that the tube current be changed accordingly for each reading, so that largely the same dose product of tube current IR, scan time t and acceleration voltage URk is produced, wherein the exponent k is selected such that, under the given conditions (pre-filter, anode material, irradiation geometry, etc.), the signal-to-noise ratio (SNR) produced is equal to the measurements at both acceleration voltages. However relatively complex power supplies and cathode constructions, which allow a correspondingly rapid modulation of the tube current, are needed for this purpose.
To avoid a tube current modulation and to achieve a largely equal dose product despite this, it is proposed, in a further variant of the fast-kV-switching method in publication U.S. Pat. No. 7,209,537, with the distribution of the readings and the reading time remaining the same, that the switch-on time of the two different acceleration voltages be varied so that the higher voltage (here 140 kV) amounts to only 20% of the switch-on time of the lower voltage (here 80 kV) related to each reading. Thus the same dose product is produced for the said acceleration voltages, taking into account the evenly distributed and alternating readings with higher and lower acceleration voltage for each acceleration voltage. However equipping the device in a way that allows a very fast switchover of the acceleration voltages is also necessary here. In addition a part of the reading time with high acceleration time is not needed for irradiation, so that a lengthening of the scanning time is produced.
Furthermore new iterative reconstruction methods are known in the prior art with which projection data obtained by stochastic scans can be used for reconstruction, such stochastic scans also being described in literature by the term “sparse sensing”, i.e. compressible or incomplete or sparse or sparsely-populated scanning. Basically in this iterative reconstruction method a distinction is made between a “compressed sensing (=CS)” reconstruction and a “prior-image constrained compressed sensing (=PICCS)” reconstruction, a reconstruction constrained by a previously known image after compressed sensing. As regards CS reconstruction, reference is made to the article entitled “FAST RECONSTRUCTION OF CT IMAGES FROM PARSIMONIOUS ANGULAR MEASUREMENTS VIA COMPRESSED SENSING” by N. S. AYBAT AND A. CHAKRABORTY. In relation to the PICCS reconstruction the publication “Prior image constrained compressed sensing (PICCS): A method to accurately reconstruct dynamic CT images from highly undersampled projection data sets” Guang-Hong Chen, a Jie Tang, and Shuai Leng, Med Phys. 2008 February; 35(2): 660-663, is cited as an example.
While CS reconstruction is undertaken exclusively with the stochastically-determined scanning data, the PICCS reconstruction uses the image information of a preceding image (prior image) of the examination object in addition to the stochastic scanning data.
To define the various technical terms rendered in German in the original we would like to state that the German term “Zwei-Energiespektren” is used in the original German document for the generally known English term “Dual-Energy”, for example in conjunction with Dual-Energy-CT or Dual-Energy-CT scanning. Furthermore English technical terms have been rendered in the original German document by the following German terms:
Single-Source=Ein-Strahlenquelle;
Dual-Source=Zwei-Strahlenquellen;
Reading=Auslesung and
Prior image=Vor-Bild.
In addition, the literal translation “komprimierte Abtastung” has been used for the English technical term “compressed sensing”, for which no suitable German technical term has been established at present, wherein it is pointed out that although this corresponds to a word-for-word translation, its sense does not correspond to the English technical term “compressed sensing”.